If you’re providing multiple themes for readers to select, follow the usage instructions at the link provided above and add one of the plugin’s template tags to your Theme’s sidebar (or wherever you want the theme links displayed).

I didn’t really want users to be able to choose their own theme (not yet, anyway), but I wanted to be able to view my own site in a prospective theme (or an experimental one) without having to change my real, live theme and without having to install an entire seperate blog just for seeing how different themes would look on mine.